1. Field of the Invention
The present invention relates generally to injection molding systems and, more particularly, to an injection molding system for injection molding a plurality of materials.
2. Description of the Related Art
Injection molding is a widely known manufacturing process used to produce a variety of parts. Injection molding involves introducing a molten material, such as a molten plastic or resin, into a cavity within a mold. The molten material hardens or cures in the mold in the shape of inner surfaces of the cavity. Then, the cured material is removed from the cavity.
A typical injection molding system includes a manifold assembly used for conveying molten material from a source such as a nozzle of an injection molding machine to a number of cavities or to multiple points within one large cavity of the mold. An example of such a manifold assembly is disclosed in U.S. Pat. No. 4,964,795 to Tooman. In that patent, the manifold assembly includes a manifold and a plurality of nozzles extending radially therefrom. Passageways are formed through the manifold and nozzles, through which the molten material may pass. The terminal end of the passageway, called a gate, is in fluid communication with the cavity of the mold.
In addition, a valve gate assembly is typically included in the injection molding system to regulate the flow of molten material through the gate into the cavity of the mold. An example of such a valve gate assembly is disclosed in U.S. Pat. No. 4,173,448 to Rees et al. In that patent, a valve gate assembly includes a valve rod or pin partially disposed within the passageway. The pin has a terminal end positioned near the gate such that it closes the gate and prevents the flow of molten material through the gate. The pin is operably attached to an actuator assembly that can move the pin axially away from the gate. As the pin moves farther away from the gate, the flow of the molten material through the gate increases.
The injection molding system often includes one or more heat sources, such as heating bands positioned on the outer surface of the nozzle and the manifold assembly. The heat sources supply heat to the molten material to maintain desired flow conditions.
It is known to provide injection molding systems with manifold assemblies having multiple flow passageways formed therein. A different molten material flows within each of the passageways, and the valve gate assembly regulates flow of these materials from the passageways into the cavity of the mold. This type of manifold assembly is used, for example, to mold parts with an outer “skin” made from one material and an inner “core” made from a different material. The valve gate assembly typically utilized includes a valve pin positioned within one of the passageways. The valve pin plugs that passageway while molten material flows through the other passageway to begin forming the outer “skin,” and after a predetermined time, the valve pin opens to begin forming the inner “core.”
One disadvantage of the above-described injection molding systems is that the flow of the molten molding materials is typically poor, thereby potentially degrading the material quality and appearance of the molded part. For instance, the valve pin creates undesirable shear on the molten material as it flows past the pin. Furthermore, pressure can build within the flowing molten material due to improperly sized flow passageways, especially near the gate. Also, one passageway is typically positioned closer than the other passageway to the heat source, and heat transfers more readily to one of the molten materials than the others. These factors can degrade the flow of the molten materials, giving users less control over the quality of the molded part. These factors also often cause an undue amount of residual molding materials to remain within the passageways after completing the molding process.
Therefore, it is desirable to provide a new injection molding system for injecting a plurality of materials in which shear on the molten materials is reduced. It is also desirable to provide an injection molding system in which the pressure of the molten materials remains more controllable and constant during the molding process. Further, it is desirable to provide an injection molding system with flow passageways arranged such that the heat source transfers heat more uniformly to the molding materials. Thus, there is a need in the art to provide an injection molding system that meets these desires.